Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction ...

17 Pages·2015·9.52 MB·English

Geophysical Journal International
Geophys.J.Int.(2015)200,1027–1043 doi:10.1093/gji/ggu454
GJIGeodynamicsandtectonics
Geophysical evidence for a transform margin offshore Western
Algeria: a witness of a subduction-transform edge propagator?
Rabia Badji,1,2,3 Philippe Charvis,2 Rabah Bracene,1 Audrey Galve,2 Madjid Badsi,1
Alessandra Ribodetti,2 Zahia Benaissa,3 Frauke Klingelhoefer,4 Mourad Medaouri1
and Marie-Odile Beslier2
1Sonatrach,Divisionexploration,BaˆtC,Avenuedu1erNovembre,B.P68M,Boumerdes,Algeria.E-mail:[email protected]
2Universite´NiceSophiaAntipolis,IRD,CNRS,ObservatoiredelaCoˆted’Azur,Ge´oazur,250rueAlbertEinstein,F-06560,SophiaAntipolis,France
3Universite´dessciencesetdelatechnologieHouariBoumediene,USTHB,FSTGAT,el-aliaAlgiers,Algeria
4Ifremer,DepartmentofmarineGeosciences,ZIdelaPointedeDiable,CS10070,Plouzane´,France
Accepted2014November19.Received2014November17;inoriginalform2014September23
SUMMARY
Forthefirsttime,adeepseismicdatasetacquiredintheframeoftheAlgerian–FrenchSPIRAL
programprovidesnewinsightsregardingtheoriginofthewesternmostAlgerianmarginand
basin.Weperformedatomographicinversionoftraveltimesalonga100-km-longwide-angle
seismicprofileshotover40oceanbottomseismometersoffshoreMostaganem(Northwestern
Algeria). The resulting velocity model and multichannel seismic reflection profiles show a
thin (3–4km thick) oceanic crust. The narrow ocean–continent transition (less than 10km
wide)isboundedbyverticalfaultsandsurmountedbyanarrowalmostcontinuousbasinfilled
withMiocene toQuaternary sediments.This faultsystem,as wellasthefaultsorganized in
anegative-flowerstructureonthecontinentside,marksamajorstrike-slipfaultsystem.The
extremelysharpvariationoftheMohodepth(upto45±3◦)beneaththecontinentalborder
underscores the absence of continental extension in this area. All these features support the
hypothesisthatthispartofthemarginfromOrantoTenes,trendingN65–N70◦E,isafossil
subduction-transform edge propagator fault, vestige of the propagation of the edge of the
GibraltarsubductionzoneduringthewestwardmigrationoftheAlbora´ndomain.
Keywords: Seismictomography;Continentalmargins:transform;Crustalstructure;Africa;
Europe.
Although this general scheme driven by subduction rollback
1 INTRODUCTION
iswidelyaccepted, manyquestionsremainunansweredregarding
The Algerian margin is part of the western Tethyan Domain. thekinematicevolutionofthewesternMediterraneanregion.The
BeginningintheLateOligocene,theclosureoftheTethyanOcean palaeoreconstruction of the Albora´n block is controversial and it
alonganorthtonorthwestdippingsubductionzoneproducedthe implies diverse types of margins (extensional or strike-slip mar-
coevalopeningofbackarcoceanicbasins(e.g.present-dayoceanic gins) and timing of seafloor emplacement (Bouillin 1986; Frizon
basinsofthewesternMediterraneandomain;Fig.1).Thecontinen- de Lamotte et al. 2000; Rosenbaum et al. 2002; Mauffret et al.
talforearcofthissubduction,ofEuropeanorigin,driftedsoutheast- 2004; Domzig et al. 2006; Schettino & Turco 2006). Three dif-
ward and collided with the North African margin (Re´hault et al. ferenttectonicscenariosexistforthepast35Ma(Chertovaetal.
1984; Lonergan & White 1997; Frizon de Lamotte et al. 2000; 2014)whichareconsistenteitherwiththewesternAlgerianmargin
Jolivet&Faccenna2000),creatinganalmostcontinuousorogenic beingapassivemargin(e.g.Jolivetetal.2009;Verge´s&Ferna`ndez
belt (named the Maghrebides in Northern Africa) spanning from 2012)orthetraceofanE–Wlithospheretearing(e.g.Rosenbaum
GibraltartoCalabria(insetsFig.1).Tothewest,theAlbora´nfore- et al. 2002; Spakman & Wortel 2004; Chertova et al. 2014; van
arcwasmigratingwestward,frommiddletolateMiocene,withthe Hinsbergenetal.2014).Thison-goingdebateislargelyduetothe
concomitantopeningoftheAlgerian–Balearicbasin(Acostaetal. lackofdeepgeophysicaldataintheAlgerianBasinandalongthe
2001; Mauffret et al. 2004) as the backarc basin of the Gibraltar Algerianmargin.
subductionzone(Rosenbaumetal.2002;Camerlenghietal.2008; OurstudyareaislocatedalongtheWesternAlgerianmargin,at
Gutscheretal.2012). thetransitionbetweenthemarginoftheAlbora´nSeaandthemargin
(cid:3)C TheAuthors2014.PublishedbyOxfordUniversityPressonbehalfofTheRoyalAstronomicalSociety. 1027
1028 R.Badjietal.
Europe 25 Ma Europe 18 Ma Europe 15 Ma
40°°Iberi8a° Alb.Kabyl.TetOhcyeaann Ib4e°ria AlBgaleeriaarinc IB.Sa.Crdoirnsiiaca 0° 4° Iberia AlgKeParriobaBvanyesil niBnec4se.0°
NIoBrtEh RAfIrAica North Africa B ale aric Isla n d s North Africa
BETICS AlgerianOceanic crust ?
Basin AlgiersGK LK
36° ALBORAN Tenes 36°
Gulf of Cadix Mostaganem ALGERIA
Oran
wedge
Tellian Units Atlas
RIF Iberian Cover Flyschs
MOROCCO Recent basins Alkapeca D.
8° 4° 0° 4°
Figure1. TectonicsketchofthewesternMediterraneanafter(Comasetal.1999)showingNeogeneextensionalbasinsandmajortectonicunits.Themajor
thrustandtheGibraltarsubductionarefrom(Gutscheretal.2012;Verge´s&Ferna`ndez2012).GK,GreatKabylia;LK,LesserKabylia.AlKaPeCaD,internal
zonesdomain.TheredrectangledelimitsthestudyarealocatedinnorthwesternAlgeria.Theinsetsshowasummaryofthereconstructionofthetectonic
evolutionofthewesternMediterraneansincetheOligocene(Rosenbaumetal.2002).
oftheAlgerianBasineastward(Fig.1).Itwaspreviouslysurveyed (Durand-Delga&Fontbote´1980;Bouillin1986;FrizondeLamotte
duringtheMARADJAcruisein2003(Domzigetal.2006)using etal.2000;Brace`ne&FrizondeLamotte2002;Fig.1).
shallowseismicimagingandswathbathymetry,andalsobyseveral Onshore,theMostaganemareafromTenestoOran(Fig.2)iscon-
industrialseismicreflectionsurveys.AspartofanAlgerian–French sideredaspartoftheexternalzones(alsonamedtheTelliandomain,
cooperativeprogram,theSPIRALcruisewasconductedonboard Fig.1).TheoutcropsconsistofMioceneandQuaternarysediments,
the R/V Atalante (Ifremer) in 2009 September–October. For the composedofmarls,limestone,sandstonesandgypsum(Perrodon
firsttimewide-angleseismic(WAS)reflection,refractiondataand 1957;Polveˆche1959;Thomas1985)whicharewellknowninthe
deepmultichannelseismic(MCS)reflectiondatawereacquiredin ChelifBasin(southofMostaganem,Fig.2).Owingtothelackof
ordertoimagethedeepstructureoftheAlgerianmargin.Themain subsurface data, the nature of the basement beneath the Miocene
aims of the survey were to explore the deep crustal structure of depositsisstillunknownintheMostaganemarea.Tothenorth,the
thecompletemargininordertobetterconstraintheopeningofthe upperpartoftheMiocenesedimentaryseriesisencounteredinthe
AlgerianBasin,toidentifypossibleindicatorsofrecentreactivation Arzew-1well,drilledinthe1970sontheshallowshelfnorthwest
ofthemargin,tocharacterizethenatureofthecrustandtoassess ofMostaganem(Medaourietal.2012;Fig.2).
thepetroleumpotentialoftheneighbouringsedimentarybasins. The opening of the Algerian Basin, between the Balearic
Thelithospherictearingandtherollbackoftheplatesubducting Islands and the Algerian margin, as the western prolongation of
beneaththeGibraltararccreatedamajorlithosphericfault,which theProvenceBasin,isstillamatterofdebate.TheNW–SEopen-
propagatedwestward.Thisfeaturewasstablewithtime,actingas ingoftheProvenceBasinstarted23Maasabackarcbasinofthe
thestrikeslipfaultboundingtheAlbora´nblockandtheAlgerian TethyansubductionbeneathAfrica(Fig.1).AccordingtoSchettino
Basintothesouth.ThisnewtypeofplateboundarynamedSTEPfor &Turco(2010),theAlgerianBasinopenedduringthelastphaseof
Subduction-TransformEdgePropagator(Govers&Wortel2005)is rotationofSardiniaandCorsica(19Ma).Analternativehypothesis
still poorly known. The western Algerian margin and basin will isanE–Wopening,from16to8Ma,associatedwiththewestward
providesomenewinsightsonthedeepstructureofthisSTEPfault. migration of the Albora´n domain (Comas et al. 1992; Platt et al.
In this paper, we present the interpretation of a joint 2-D re- 1998;Comasetal.1999;Rosenbaumetal.2002;Mauffretetal.
fraction and wide-angle reflection traveltime tomography model 2004,2007;Booth-Reaetal.2007;Medaourietal.2014;Fig.1).
along a profile shot offshore Mostaganem, together with regional This second hypothesis is preferred by many authors even if the
SPIRAL and industrial MCS sections, and available bathymetric distance of the westward migration of the Albora´n domain is de-
data,inordertoimagethedeepstructureoftheMostaganemmar- batedandrangesfrom∼200to∼700km(Michardetal.2002;Platt
ginsegment. &Houseman2003;Duggenetal.2004;Mauffretetal.2004;Platt
etal.2006;Medaourietal.2014).InthiscasethewestAlgerianand
theMoroccanmarginswerestructuredbytheretreatofthewest-
ward migration of the southern edge of the Gibraltar subduction
zone(Lonergan&White1997;Faccennaetal.2001;Chalouan&
2 GEODYNAMIC FRAMEWORK OF THE
Michard2004;Govers&Wortel2005).
ALGERIAN BASIN
TherecentlyacquiredseismicdataoftheSPIRALprojectwill
ThesouthernMediterraneanseaisboundedinnorthernAlgeriaby providenewconstrainsontheopeningoftheAlgerianBasin.The
an Alpine orogenic belt resulting from the collision of continen- structural direction of the Algerian margin changes from N65◦E
tal fragments of European origin with the North African margin to N70◦E west of Tenes to N85◦E east of Tenes (Fig. 2). To the
TheAlgerian–FrenchSPIRALprogram 1029
Figure2. BathymetricandtopographicmapofthewesternAlgerianmargin.Seafloorbathymetry(50mdigitalelevationmodel)derivedfromsatellitealtimetry
andfromswathbathymetricdatacollectedduringtheMARADJAcruise(Domzigetal.2006).Redlinesmarkmultichannelseismicprofilesandblackdots
locationoftheseismometersonlandandatsea.ThewhitedotsarelabelledandindicatethepositionsofseismometersshowninFig.3.GreylineA–Bmarks
thebathymetricprofileshownininsetwhichhighlightsthesteepcontinentalslope.ARZ-1isacommercialwelldrilllocatedinArzewbay.
east the deep structure of the margin exhibits a large continental tervals.Theseismicsourcearrayforthewide-angleseismicprofile
tiltedblock,theKhayr-al-DinBank,consistentwithariftedmargin hadatotalvolumeof146l(8909in3)andwascomposedofeight
whereastheverynarrow(10kmorless)ocean–continenttransition airgunswithanindividualvolumeof16l,andtwoairgunswitha
(OCT)suggestthemarginispossiblyapurelystrike-sliptypemargin volumeof9l.Allseafloorinstrumentdatawerecorrectedforthe
formedasaSTEPfaultsystem(Lepreˆtreetal.2011,2013).The clockdriftduringtherecordingperiod.Thespatialdriftbetweenthe
authors propose a multiphased emplacement of the margin with deploymentlocationandtheseafloorlocationwasestimatedfrom
the∼NW–SEextensionassociatedwiththeopeningoftheCentral theinversionofdirectwaterwavetraveltimes.Thepre-processing
AlgerianBasinandamorerecenttranscurrentepisodeinducedby sequenceofdataincludesfrequencyfilters,aminimumphasespec-
thewestwardmigrationoftheAlbora´nblock. traldeconvolutionandanautomaticgaincontrol(seeFigs3andS1
Whereas most of the Algerian margin undergoes present-day formoredetails).
NW–SE compression (Stich 2003), there is no clear offshore ev-
idence for active deformation related to this compression along
thewesternmostpartoftheAlgerianmargin(Domzigetal.2006).
3.2 MCSdata
Onshore,earthquakefocalmechanismssupportaNW–SEcompres-
sionalstressintheOran–Mascara–ElAsnam(namedChelifsince TwoMCSsectionswereshotduringtheSPIRALcruiseacrossthe
the M 7.2 El Asnam earthquake in 1980) area (Meghraoui et al. marginoffshoreMostaganem.SectionSpi02iscoincidentwiththe
1986,1996;Yieldingetal.1989). WASprofilepreviouslydescribedwhereasprofileSpi04(∼80km
Inthisgeodynamicframework,theMostaganemarearepresents long)isparallelbutlocated104kmtotheeast(Fig.2).Theseismic
a key zone, which can potentially provide insights into the struc- lines were shot using an array of 13 airguns with a total volume
tureandevolutionoftheWesternMediterraneanfromMioceneto of3099in3.Inordertostrengthenlow-frequencies,thesourcewas
present. tunedinsingle-bubblemode(Avediketal.1993).Thistechnique
enhanceslowfrequenciesandtherebyimprovessignalpenetration
andsubsaltdepositsimaging.Theseismicsourcewasfiredevery
20s,leadingtoashotspacingof∼50malongbothlines.Thedata
3 DATA SET
were recordedusinga4.5-km-long streamer,360channel groups
providing45foldcoverageand6.25mCDP(commondepthpoint)
3.1 SPIRALwide-angleseismicdata
spacing.Therecordinglengthwas17secondstwo-waytraveltime
Toprobethecrustalstructureofthemargin,a∼100-km-longwide- (stwt)witha4mssamplingrate.
angleseismicprofileperpendiculartotheshorewasacquireddur- To further improve the quality of the seismic images issued
ing the SPIRAL cruise (2009 October–November). 35 MicrOBS fromtheon-boardinitialprocessing,weappliedthefollowingpro-
ocean bottom seismometers (OBS; Auffret et al. 2004) and five cessing sequence to the data using Geocluster software (a CGG
(OBH),belongingtoIfremerandtheUniversityofBretagneOcci- Veritaspackage):(1)azero-phasingofsinglebubblewavelet,(2)
dentale(UBO)weredeployedoffshoreMostaganem,fromtheN/O aninverseQfilter(phaseonly)toremovethenon-stationaryphase
Atalante,at1.9kmintervalsinaverage(Fig.2).Severallandsta- componentsofthedata(Varelaetal.1993),(3)apre-stacksurface
tions were deployed, but only few of them recorded useful data. multiple modelling also known as SRME (surface related mul-
Inthispaper,weusedlandstationsLS2andLS3located,respec- tiple elimination), followed by an adaptive subtraction from the
tively,at14and21kmfromthesouthernmostOBS(Fig.2).These data (Berkhout & Verschuur 1997; Verschuur & Berkhout 1997),
seismometersrecorded489shotsevery60sleadingto∼150min- (4) a velocity analysis, (5) a second pass of multiple attenuation
1030 R.Badjietal.
in Medaouri et al. (2014). Several sections available in the study
areawereusedtodiscusstheextensionofthestructuredescribed
along profile Spi02 and to discuss the Mostaganem margin deep
structure.
4 JOINT REFRACTION AND
REFLECTION TRAVELTIME
TOMOGRAPHY
4.1 Methodology
Usingrefractedfirstarrivalsandreflectionsobservedonwide-angle
seismicdata,weconstructa2-Dvelocitymodelofthesedimentary
layers,theigneouscrustandthegeometryoftheMoho,usingthe
jointrefractionandreflectiontraveltimetomographiccodetomo2d
(Korenaga et al. 2000). Refracted first arrivals as well as reflec-
tionsfromtheMoho(PmP)wereidentifiedandcorrelatedbetween
the different OBS sections (Fig. 3). The reflection from the top
ofthebasement(Prb)wasalsoidentifiedbyacarefulcorrelation
between each OBS and the coincident MCS profile at null offset
(Fig. S2). Approximately 11 000 arrivals were picked. The pick-
inguncertaintyassociatedtoPsedandPgarrivaltimes(wavesre-
fracted, respectively, in the sedimentary cover and in the igneous
crust)werebetween16ms(representingapproximatelytwicethe
samplingrate)and95ms(representingapproximatelythedominant
periodofarrivalsatfaroffsets)forthearrivalswithhighandlow
qualitysignal-to-noiseratio,respectively.Thepickinguncertainties
forthereflectedsecondarrivalsweresetto100mswhichrepresents
approximatelyoneperiodofthesignal.
The2-Dvelocityfieldisparameterizedasashearedmeshhanging
beneaththeseafloor-landsurface.Aninitial1-Daveragevelocity
modelisdeterminedbyforwardmodellingandextrapolatedalong
the 2-D line beneath the topography (Korenaga et al. 2000). The
codeallowsthesimultaneousinversionoftraveltimesfromrefracted
firstarrivalsandfromareflectedphase,toobtainavelocitymodel
andthegeometryofafloatingreflectordefinedasanarrayoflinear
segmentsindependentofthevelocitygrid(Korenagaetal.2000).
To obtain the best 2-D velocity model we performed a top-to-
bottom layer stripping strategy consisting of adding the data se-
quentiallyasdescribedinSallare`setal.(2011,2013a).Duringthis
procedure the damping was applied automatically if the velocity
perturbation exceeds the maximum allowed value (10 per cent in
ourcase;Korenagaetal.2000).
Westartedwiththesedimentphasesalone(Psedrefractedinthe
Figure3. Examplesofrecordedwide-angleseismicsections.Eachsection sedimentarycoverandPrbreflectedfromtheacousticbasement).
wasprocessedincludingadebiasfilter,aminimumphasespectraldecon- Duringthisfirststepoftheinversiontheinitialbasementinterface
volution,a4–15HzButterworthband-passfilteranda2swindowlength waschosenasahorizontalreflectorat5kmdepth.Inasecondstep
automaticgaincontrol(AGC).Thesectionarepresentedwitha7kms−1
weinvertedthephaserefractedinthebasement(Pg)andthephase
velocityreduction.Foreachsection(LS3,landstation3,andOBS-01,18
reflectedfromtheMoho(PmP).Forthissecondphase,theinitial
and27fromtoptobottom)traveltimescalculatedinthefinalmodel(Figs4
velocitymodelwastheresultoftheinversionofsedimentphases
and7)aresuperimposed.Green:Psedrefractedinthesedimentarycover,
abovethetopofthebasementandthemodelresultingfromtheinitial
red: Pg refracted in the crust, orange: Pn refracted in the upper mantle,
violet:Prbreflectedfromthetopofthebasementandblue:PmPreflected extrapolationofthe1-Daveragevelocitymodelbelowit.Because
fromtheMoho. ofthesteeptopographyoftheMoho,theinitialmodelwasa2-D
dippinginterfaceobtainedbyforwardmodelling(Fig.S3).Inthe
using a predictive-subtraction process, (6) a Kirchhoff pre-stack laststepweusedthecrustalvelocitymodelandaconstantvelocity
time migration, (7) an external mute, (8) a stack, (9) a 5–60 Hz of8.2kms−1intheuppermantleasinitialmodelandincorporated
bandpassfilterand(10)a500msautomaticgaincontrol. Pnphasesintheinversiontodefinetheuppermostmantlevelocity
InthewesternpartoftheAlgerianoffshore,Sonatrachacquired field.BecausePnphasesareobservedonlyonOBS1andonland
a set of seismic reflection data (Cope 2003) using a 6-km-long stationsatthesouthernendoftheprofile(Fig.3),velocitiesinthe
seismicstreamer(480channelsof12.5m)anda3000in3 air-gun uppermantleareonlypoorlyconstrainedandwillnotbediscussed
sourcefiredat50minterval.Theprocessingsequenceisdescribed inthispaper.
TheAlgerian–FrenchSPIRALprogram 1031
Figure4. Resultoftheinversionofthewide-angleseismictraveltimesbasedonalayerstrategy(Sallare`setal.2011,2013a).Themodelwasobtainedby
theinversionofthePs,Prb,Pg,PmPandPnphaseswiththejointrefraction,reflectiontraveltimetomographytomo2dcode(Korenagaetal.2000).(a)Final
velocitymodelofthecrustanduppermantle.ThegeometryofthetopofthebasementandtheMohoareindicated,respectively,inwhitedashedlineand
inblack.RaysreflectedfromtheMohoareindicatedingrey.(b)Thederivativeweightsum(DWS)whichindicatestheraydensityclearlyshowsthatthe
uppermantleispoorlysampledbecauseonlyOBS1andland-stations2and3recordedPnphase.Thereforethevelocityintheuppermantleisnotproperly
constrained.RedcirclesdisplaythelocationoftheOBSs,OBHsandlandstationsusedinthiswork.Numbersinbluerefertoseismometers.
Akeyadvantageofthisstrategyisthatitallowsincludingsharp
4.2 Analysisoftheuncertaintiesofthemodel
velocitycontrastsacrossgeologicalboundaries,ratherthanunreal-
isticallysmoothedvelocitygradientsrequiredintraveltimetomog- Inordertoestimateuncertaintiesofthefinalmodelweperformeda
raphy (Sallare`s et al. 2011). On the other hand it requires the a MonteCarloanalysis(Tarantola1987).Practicallyweinverted100
prioriinterpretationoftheoriginoftheobservedfirstarrivalseis- setsoftraveltimes,withrandomGaussiannoiseaddedtotheinitial
micphase,whichmightbepartlysubjective.Becauseweobserved dataset(±50msforthephaseerrorsand±10msfortheshoterrors)
mostly the pre-critical and critical PmP in our data set (Fig. 3) it with100randominitialmodels(Korenagaetal.2000;Sallare`setal.
isratherdifficulttodetermineaccuratelywhenfirstarrivalschange 2003). The set of 100 1-D initial velocity models is constructed
from Pg to Pn. We nevertheless assume that beyond the critical by randomly perturbing velocity and reflector depth in the initial
distance,markedbythemaximumamplitudeofthePmP,thefirst modelswithinreasonablebounds.Practically,weaddedrandomly
arrivalwerePnrefractedintheuppermantle(seesectionLS3and varyingvelocities(±0.5kms−1)toseveralnodesatrepresentative
OBS1inFig.3). depthsofthe1-Daveragemodel(Fig.S3).The1-Dvelocitymodelis
The best velocity model is obtained after 10 iterations for the thenextrapolatedalongthe2-Dmodel.Similarly,weaddedrandom
inversionofeachlayer.Thefinalrmsis64msandthechi-square depth (±1km) to the ends of the 2-D initial Moho, and linearly
value(χ2)1.7forthecrustandmantlearrivals(Fig.4andTable1). interpolatedthisvariationateachnodeoftheinterface(Fig.S3).
Theupper8kmofthecrustarewellsampledfromkm-0tokm-80 Finally, a 2-D inversion is performed for each random initial
whereasthelowercrust,theuppermantle,andthesouthernpartof modelwithaspecificdatasetwithGaussiannoiseadded.Wecom-
themodelarepoorlysamplebyrays(Fig.4).Tofurtherassessour putetheaverageofalltheMonteCarlorealizations(Fig.5a)and
finalvelocitymodelweevaluateditsreliabilityandresolutionusing thestandarddeviationsofvelocityanddepthparameters(Fig.5b).
aMonteCarlosimulationandcheckerboardtests,respectively. The standard deviations of such an ensemble can be interpreted
1032 R.Badjietal.
Table1. Parametersusedduringtheinversion.Severalvaluesoftheinversionparametersweretestedandthistable
summarizethevaluesprovidingthebestfinalmodel.
Model parameters
Nb of Number of picks Model Grid
instruments dimensions
Refracted first PmP X Z (km) Horizontal number Vertical number
arrivals (km) of nodes of nodes
42 10200 1200 110 40 221 40
Inversion parameters
Nb
Smoothing Damping Correlation length
iterations
in in in in
Horizontal Horizontal Vertical Vertical
depth velocity depth velocity
(top) (bottom) (top) (bottom)
(%) (%) (%) (%)
Sediments 10 8 55 10 10 1.5 3.0 1.0 4.0
Crust and 10 8 55 10 10 3.0 10.0 1.0 6.0
mantle
Results of the inversion
RMS of refracted phases (s) RMS of reflected phases (s) chi-square
Sediments 0.064 0.183 4.30
Crust and mantle 0.064 0.111 1.69
as a measure of model uncertainties. The derivative weight sum variationrelatedtosaltdiapirsarealmostfadedawayinthemean
(DWS)whichisthecolumnsumvectorofthevelocitykernelpro- velocity model). Throughout the rest of this paper we will use
videsinformationonthelinearsensitivityoftheinversion(Fig.5c; the best velocity model obtained by the layer stripping approach
Korenagaetal.2000). (Figs4aand7a).Itexhibitsastrongvariationofthecrustalvelocity
The velocity uncertainties are generally lower than 0.2kms−1, structure between the deep basin (km-0–42), where the seismic
except on-land due to the lack of crossing rays and a low DWS, velocityreaches6.7kms−1 at9kmdepth,atthebaseofthecrust,
andlocallyinthelowercrustbetweenkm-40and50.Inthislatter and the margin (km-55–90) where a similar velocity is reached
casethereispossiblyatrade-offbetweentheMohodepthandthe at ∼15km depth. In order to discuss the nature of the crust, we
velocityinthelowercrust. comparethevelocity–depthcurvesextractedatdifferentlocations
The Moho depth is well constrained betweenkm-0 andkm-20 inourfinalmodeltoreferencevelocity–depthcurvesfortheoceanic
(9.6 ± 0.34km) and betweenkm-45 andkm-55 (uncertainty of crust(Whiteetal.1992)andforthecontinentalcrust(Christensen
±0.45km)whereitisdeepeningwithanangleofupto45◦(Fig.5). &Mooney1995;Figs7b–d).Basedonthisanalysis,threedomains
canbedefined:theoceanicdomain,thecontinentalmarginandthe
OCTzone.
4.3 Modelresolutionanalysis
Checkerboardisaconventionalproceduretoinvestigatetheresolv-
4.4.1 Theoceanicdomain
ingpowerofthedataset.Toperformcheckerboardtests,synthetic
traveltimesarecomputedinaperturbedvelocitymodelconsisting This area is characterized by a relatively flat bathymetry
of a lateral alternating squared pattern added to our final model. (∼2700m) from OBS19 towards the northern end of the profile
Theresolutionofourfinalmodelisevaluatedbythecapacityofthe (km-0–42;Figs2and7a).
inversiontorecovertheperturbationpatternusingourfinalvelocity Along the coincident, time-migrated MCS profile (Fig. 8) the
modelasthestartingmodel(Zelt1998;Zeltetal.2004).Inorderto bottom of the sedimentary pile is an energetic seismic reflector
obtainagoodcartographyofverticalresolutioninourfinalmodel, locatedbetween5.2(atthenorthwesternendoftheprofile)and5.8
weadaptedandappliedthemethoddevelopedbyZelt(1998)for stwt(atkm-40)andislocatedatadepthof6–7kminourvelocity
a3-Dtomographytoour2-Ddataset.Wecomputed40checker- model(Figs4aand7a).Consequentlythesedimentarycoveris3.3–
board models with velocity anomalies ranging from 3×3km2 to 4.3-km-thick with seismic velocities up to 4.8kms−1 at its base.
20×20km2andamplitudeof±6percent.Theresultswerecompiled ThesehighvelocitiesareconsistentwiththepresenceofMessinian
toobtaintheabilityofthedatatoresolvefeaturesofagivensize saltlayersknownthroughouttheAlgerianBasin(Lofietal.2011).
accordingtodepth(Fig.6).Inthefirstkilometresofthemodel,the The 3.0kms−1 isovelocity contours outline the geometry of salt
verticalresolutionisaslowas3kmshowingthatweindeedresolve diapirs(Figs4aand8).
someofthesaltdiapirs(Fig.4a).Above10kmdepthandbelowthe Belowthisreflector,velocity–depthcurvesfortheigneouscrust
OBSline,wehaveabetterresolutionthan7–8km.Aresolutionof fitwithintheenvelopedefinedfortheoceaniccrust(Whiteetal.
lessthan15kmisobtainedforthedeepestpointofraypenetration 1992)but,atthesamedepth,itsaveragevelocityishigherthanthe
belowtheOBSlineandfordistancealongtheprofilehigherthan averageoceanicvelocity(Fig.7b).Only2OBSsectionsshowclear
80kmduetoalackofraycrossingforthetwoisolatedlandstations. PmP arrivals (e.g. OBS 27 in Figs 3 and 4a), whose traveltimes
inversiondefinestheMohoat10±0.5km(Figs4–5).
Althoughwide-anglereflection(PmP)fromtheMohoaresparse,
4.4 Seismicvelocitystructure
weaknear-verticalreflectionsappearfromplacetoplace,between
ThemeanvelocitymodelobtainedfromtheMonteCarlosimulation 6.8 and 7.0 stwt, along MCS profile Spi02 (Fig. 8), and a clear
(Fig.5a)doesnotdiffersignificantlyfromthebestmodel(Fig.4a) reflective band appears along MCS profile P4, an industrial pro-
exceptitissmoother,especiallyinthesedimentarycover(velocity file located near profile Spi02, between 6.5 and 7.0 stwt (Fig. 9).
TheAlgerian–FrenchSPIRALprogram 1033
Figure5. ResultoftheMonteCarlosimulation.(a)MeanvelocitymodelandmeanlocationoftheMoho(thickblackline)obtainedbyaveragingthe100
finalmodelsresultingfromtheMonteCarlosimulation.(b)FinalstandarddeviationvaluesforthePseismicvelocityandtheMohogeometry(pinkband),as
aresultofthestatisticaluncertaintyanalysis.(c)Derivativeweightsum(DWS)valuesoftheabovetomographicmodel.Redcirclesdisplaythelocationofthe
OBSs,OBHsandlandstationsusedinthiswork.Numbersinbluerefertoseismometers.Thewhitedashedlineindicatesthelocationofthetopoftheacoustic
basement.Themantleisnotincludedinthissimulation.
Thesedifferencesarepossiblyfrequencydependent:thereflections servedonMCSlinescorrelatewellwiththeMohoinferredfrom
areweakalongSpi02(peakfrequencyof20Hz)whereastheyare thewide-anglereflectionsidentifiedonOBS40andOBS27record
strongeralongprofileL3(peakfrequencyof40Hz;Figs8and9). sections(Figs3,4aand8)andlikelyrepresentreflectionsfromthe
Atwide-angle,theMohomaybepoorlyreflectivebecausethepeak oceanicMoho.Tothesouth,PmPphasesrecordedbyOBSlocated
frequency is as low as 8 Hz for the wide-angle seismic data but onthemargin(Fig.3)constraintheMohodepthto∼10kmatkm-
alsopossiblybecauseofpoorsignal-to-noiseratioonseveralOBS 40 (Fig. 4a) which is consistent laterally with our interpretation.
sections (Fig. S1). Nevertheless, the near vertical reflections ob- WechosetomodeltheoceanicMohoasafirstorderdiscontinuity
1034 R.Badjietal.
Figure6. Resolutionofthefirstarrivaltomographicmodel.Eighttypesofcheckerboardmodelswerecalculatedforeachvelocityanomalysize(3×3km2,
6×6km2,10×10km2,15×15km2and20×20km2),withthepatternbeingspatiallyshiftedandrotatedby45◦andthepolarityofanomaliesreversed.The
semblancebetweentheexactandrecoveredcheckerboardisthencalculated(Zelt1998)foreachcheckerboardmodelandanaveragesemblanceforeach
anomalysizeisderived.Partofthemodelwithsemblancehigherthan0.7areconsideredwellresolved.Finallyaninterpolationismadetoestimateateach
nodeofthemodel,theanomalysizethatwillgiveasemblancehigherorequalto0.7.
Figure7. Interpretationofthevelocitymodel.(a)Finalvelocity–depthmodelresultingfromthetraveltimeinversion(sameasinFig.4a)withnovertical
exaggeration. OBS and land station LS locations are indicated by red circles, isovelocity contours are represented every 0.5kms−1 and labelled every
1kms−1.Thickgreylinesrepresentthetopoftheacousticbasement,andtheMohodiscontinuity.OCT,ocean–continenttransition.The1-Dvelocity–depth
profilesbelowthetopofthebasementwereextractedat1kmintervalalongthevelocitymodelatdistancesof(b)30–41km(c)42–48kmand(d)49–65km.
Theyellowarearepresentsavelocitycompilationforextendedcontinentalcrustextractedfrom(Christensen&Mooney1995),thelightgreenarearepresents
avelocitycompilationforAtlanticoceaniccrustfrom(Whiteetal.1992)andtheblueareaacompilationofexhumedmantleregionsoffshoreWestIberia
(Sallare`setal.2013b).
butitislikelyhighlyvariablealongtheprofile.Insomeplace,the velocities range from 5.0 to 6.5kms−1, and the vertical velocity
apparent lack of PmP and the reflectivity observed at normal in- gradientfrom1.9kms−1km−1atthetopto0.4kms−1km−1atthe
cidencesuggestthecrusttomantletransitionissmoothwiththin bottom.Thelowercrustiswithis1–2kmthickwithvelocitiesfrom
embeddedlayers. 6.5to6.8kms−1(Fig.7b).
AsfirstarrivalPntraveltimesareobservedonlyonlandstations
and on OBS1 record sections (Fig. 3), the velocity in the upper
mantle cannot be determined accurately but it is probably rather
high(8.6kms−1inaverage). 4.4.2 Thecontinentalmargindomain
These observations support a Moho depth of 9–10km beneath In this domain, the thickness of the sedimentary cover ranges
theoceanicbasin,whichleadstoathicknessoftheigneouscrust from ∼0 (at 60km near the outer edge of the continental slope)
aslowas3–4km.Inthetwoupperkilometresofthecrust,seismic to3kmthickwithseismicvelocitiesreaching4.0kms−1atitsbase
TheAlgerian–FrenchSPIRALprogram 1035
Figure8. TimemigratedsectionSpi02superimposedonthefinalvelocitymodel(Fig.4a)convertedintwowaytime.OBSlocationsareindicatedbyred
circles.TheyellowlineindicatesthetopoftheacousticbasementreflectorinterpretedontheMCSdata.Thelightwhitelinesarevelocitycontours3.0,4.0,
5.0,6.0and7.0kms−1.Thesharpvelocitycontrastatthebaseofthecrustcoincideswiththeweakreflections(highlightedbywhitedots)recordedat≈6.8–7
stwtwhichareinterpretedastheMohodiscontinuity.
Figure9. TimemigratedseismicsectionP4(seeFig.2forlocation).ThislinelocatedinthevicinityofprofileSpi02clearlyshowsareflectivebandbetween
6.5and7.0stwt(indicatedbybrackets)whichlikelyindicatestheoceanicMohoatatimesimilartothatoftheMohoreflectionsobservedonprofileSpi02
(Fig.8).
whichlikelycorrespondstorecentdetriticsediments(Domzigetal. Inthisarea,OBSsrecordedPmPreflections,allowingconstrain-
2006;Fig.8). ingtheMohodiscontinuitydepth,whichincreasesrapidlyfrom11
From OBS-16 (km-50, Fig. 4b) to the end of the model on- to14kmwithadipof∼27◦.Tothesouth,beneaththecontinental
land, the velocity model exhibits a low vertical velocity gradient margin,thedipoftheMohohasavalueashighas∼45◦(Figs4aand
(0.25kms−1 km−1 in average) that fits well within the range of 7a).TheuncertaintyontheMohodepthinferredfromtheMonte
thinnedcontinentalcrustvelocities(Fig.7d;Christensen&Mooney Carlosimulationis±0.45kmwhichleadstoanuncertaintylower
1995).Continentalcrustextendsbeneaththecontinentalslopeand than±3◦ontheMohodip.
plateau.TheMohodiscontinuity,invertedasareflectorfromPmP
arrivals(Figs4and5),dipsrapidlytowardsthesouthfrom∼15km
depth beneath OBS-16 (km-50) to more than 20km towards the 5 INTERPRETATION OF SEISMIC
continent. REFLECTION LINES
Thestudyareaiscoveredbyseveralseismicsurveysconductedby
4.4.3 Thetransitionzone SonatrachorintheframeworkofthescientificAlgerian–Frenchco-
operationliketheMaradjaproject(e.g.Domzigetal.2006)orthe
TheOCTislocatedbetweenOBSs16–20(km-50–42).Thisareais
characterizedbya∼4-km-thicksedimentarysequencewithseismic SPIRALproject(Lepreˆtreetal.2013;Medaourietal.2014).Two
velocitiesof4.5kms−1atitsbase(Figs7aand8). sectionsSpi02andSpi04arechosentoillustratetheMostaganem
generalstructureframework(Figs10and11).Onlylarge-scalefea-
Thiszoneischaracterizedby1-Dvelocity–depthcurveswhich
tureswillbediscussedhere,concerningtheOCT,thesedimentary
range between oceanic and continental velocities (Fig. 7c; White
coverofthedeepbasinandthemajortectonicfaults.
etal.1992;Christensen&Mooney1995).Thewidthofthisregion
islessthan10km.Itischaracterizedbyastrongverticalvelocity
gradientintheuppercrust(0.6kms−1 km−1)andvelocitiesinthe
5.1 Oceanicdomain
rangeofcontinentalvelocitiesinthelowercrust(Fig.7d).Thelower
crustappearsasalowvelocityzone(Fig.7c)butitispossiblyan Basedonthecorrelationbetweenseismicprofilesandvariouswells
artefactasitcoincideswithhighstandarddeviationintheMonte (Medaouri et al. 2012), it is possible to assign an age to the ma-
Carloanalysis(Fig.5b). jorsedimentaryseriesobservedthroughoutthearea.Theseseries
1036 R.Badjietal.
Figure10. TimemigratedMCSprofileSpi02(seelocationinFig.2).(a)Uninterpretedsection.(b)Interpretedsectionshowingmajorstratigraphicand
structuralunitsoftheAlgerianmarginoffshoreMostaganem.Thelimitsoftheoceaniccrust,thecontinentalcrustandtheOCTarefromthetomographic
inversionofcoincidentwide-angleseismicline.G:narrowasymmetricgrabendevelopedattheOCT.Itislimitedbytwomajorverticalcrustalfaults,the
northernonebeingunderlinedbyaprominentsaltstructure(AmeurDiapir).FS:faultsorganizedinanegativeflowerstructure.Theredlinemarkedthetopof
theacousticbasement.
correspond to the succession classically observed in Western deposits(Messinianupperevaporites,PlioceneandQuaternaryse-
Mediterraneanandpreviouslydescribedbyseveralauthors(Mauf- ries)andcreatesanescarpmentontheseafloorwhichisvisibleas
fret et al. 2004; Domzig et al. 2006). Along Spi02, Miocene analmostcontinuouslinearreliefonthebathymetricmap(Fig.12).
sedimentary deposits lie directly on top of the oceanic basement At depth under the salt ridge, the base of MU and the top of the
(Fig. 10). The thickness of the evaporitic series associated to the basement are shifted of 0.1–0.5 stwt (km-43, Fig. 10 andkm-73
Messiniansalinitycrisis,namelythemobileunit(MU)andtheup- Fig.11),whichcanonlybeexplainedbycrustal-scalefaulting.It
perunit(UU)varieslaterallyastheselayersarestronglyaffectedby isknownexperimentallythatviscousdiapirsareobservedtoform
diapiricdeformation(Lofietal.2011).Messiniandiapirscrosscut above basement faults (e.g. Koyi et al. 1993). Consequently, we
theoverburdenPliocenesedimentaryserieswhicharethendiscon- assumethatthismajorsaltridge,whichisobservedonallthedeep
tinuous. These series are also affected by folds and faults related seismiclines,outlinesamajorcrustalfaultlocatedattheboundary
tothesalttectonics,inparticularbysmall-offsetfaultsatthetop between the oceanic crust and the continental crust (Figs 10 and
ofdiapirs.ThesedeformationsaresealedbytheupperQuaternary 11).Thefaultand/ortheAmeursaltridgeaffecttheseafloorand
deposits,supportingadiapirictectonicphaserangingfromPliocene havelikelybeenactiveuntilrecently.
tomid-Quaternary,exceptforthesoutherndiapirs,suchastheone
observedatkm-43onSpi02,whichrisesuptothesurface(Fig.10).
Thesouthernlimitoftheoceanicdomainidentifiedfromtheto-
5.2 Continentalmarginandtransitionzone
mographic model (km-42) coincides with the southernmost large
diapirobservedalonglineSpi02(Figs8–10).Asimilarlargediapir Along line Spi02, a narrow (10-km-wide) sedimentary basin de-
is observed atkm-72 along profile Spi04 (Fig. 11). This salt fea- veloped on top of the OCT (labelled G,km-42–52, Fig. 10). Its
ture,namedAmeurdiapir,crosscutsthewholeuppersedimentary northern flank is the major fault described before and marked by